DENVER -- The defending World Series champion San Francisco Giants are sitting atop the National League West standings.

But there is a warning sign that can't be ignored: A struggling rotation, which has been the foundation for a team that has won two of the past three World Series.

The Giants' starting five went into Saturday with a 4.69 ERA, 13th in the National League, ahead of only San Diego (5.04) and Milwaukee (5.14). Even the Rockies' rotation has a lower ERA (4.57).

Reason for concern?

Over the past decade, the Giants have gone into June with a rotation ERA in excess of 4.00 only four times, and while in 2004, despite a 4.44 rotation ERA, the Giants did finish second in the NL West with a 91-71 record, the three more recent early-season rotation struggles have been indications of season-long problems.

The Giants were 72-90 in 2008, when the rotation had a 4.37 ERA in April/May; 76-85 with an April/May rotation ERA of 4.47 in '06, 75-87 in '05, when the rotation ERA in the first two months of the season was 4.68.

Giants manager Bruce Bochy, who has made a successful career out of never letting the public see him sweat, remains confident in his five-man alignment of Madison Bumgarner, Matt Cain, Barry Zito, Tim Lincecum and Ryan Vogelsong.

What can't be ignored, however, is that the Giants have been the last team standing two of the past three years, meaning they had an extra month of wear and tear on the pitching staff.

"You always wonder," Bochy said. "[Pitching coach] Dave Righetti and I have talked about it. We try to keep a watchful eye. There's a high level [of innings] when you [go deep in the playoffs] two of three years. We try to give a break when we can."

Bumgarner has a rotation-best 3.09 ERA, and that was inflated from 2.18 on Friday night when he got caught up in a Coors Field adventure, and gave up a career-high nine runs, seven earned, in 4 2/3 innings. He's thrown 99 pitches or more in each start, and the 10-9 loss to the Rockies has been the only time he didn't pitch into the sixth inning. He has allowed two or fewer runs in seven of nine starts.

Zito goes into a Sunday start with a 3.40 ERA, having allowed one or no runs in six of his eight starts. Lincecum, who has a 4.07 ERA, went into his Saturday night start having worked seven shutout innings against Atlanta on Sunday, and Cain's ERA 5.43 ERA is still bloated from a nine-run, 3 2/3 inning effort in his second start of the season against St. Louis. The Giants have won his last four starts.

Then there is Vogelsong, who turns 36 on July 22 and is the elder statesman of the rotation. He has an 8.06 ERA, highest of any qualifier in the big leagues, and that doesn't account for seven unearned runs he has given up to go along with 59 hits and 17 walks in 41 1/3 innings. Including the postseason, he worked a career-high 214 1/3 innings last year.

And there have to be questions about whether he physically recovered from the demands of 2012, especially after also pitching for the United States team in the World Baseball Classic this year, which meant he had to speed up his conditioning program to be ready for competitive action by early March, a month ahead of his fellow Giants starters.

IT SEEMS WORTH noting that the play that sparked the recent public discussions about the need for increased usage of replays was one that involved the use of replay (in which a double that should have been a home run was left a double) and a misinterpretation of the rule requiring a pitcher to face at least one batter once he enters a game. Increased replays would not have impacted either play.

NOT SINCE 1948 have the Chicago White Sox and Cubs finished in last place in the same season. They were both in last place in their divisions on Wednesday, but the White Sox were a percentage point ahead of Minnesota in the AL Central, and the Cubs a half-game ahead of Milwaukee in the NL Central on Saturday morning.

HOUSTON LOST 30 of its first 40 games for the first time in its 52-year existence. The worst record for a team in the first 40 games was 6-34 by the 1988 Baltimore Orioles, who lost a record 21 in a row to open that season. The previous record was a 0-13 start by the 1904 Washington Senators and 1920 Detroit Tigers.

MARK BUEHRLE gave up five runs in six innings of Toronto's 5-0 loss to the Yankees on Friday, leaving him with a 0-8 record in his last 11 starts against the Bombers, and with a 1-10 lifetime record, the worst winning percentage of any active pitcher against the Yankees.

OUT OF LEFT FIELD FACTOID OF THE WEEK
Tuesday was just the fourth time in Major League history that seven Cy Young Award winners -- Bartolo Colon, R.A. Dickey, Felix Hernandez, Clayton Kershaw, Jake Peavy, CC Sabathia and Barry Zito -- started on the same day, according to the Elias Sports Bureau. The three other times were April 21, 1974 (Vida Blue, Steve Carlton, Mike Cuellar, Bob Gibson, Jim Palmer, Jim Perry and Tom Seaver), and on April 5 and April 10, 1993 (Roger Clemens, Doug Drabek, Dwight Gooden, Orel Hershiser, Greg Maddux, Rick Sutcliffe and Bob Welch).

Tracy Ringolsby is a columnist for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.